Extended interaction structures of the prior art have several cavities of the doubly re-entrant type with electromagnetic fields strongly coupled by slots in the walls (see A. S. Gilmore, Jr., "Microwave Tubes", Artech House, Norwood, Mass., 1986, Ch. 11, A. Staprans, E. W. McCune, and J. A. Ruetz, "High-Power Linear-Beam Tubes", Proc. IEEE, vol. 61, pp 299-330, 1973; M. Chodorow and T. Wessel-Berg, "High-Efficiency Klystron with Distributed Interaction", IRE Trans. Electron Devices, pp. 44-55, 1961). The electrons interact with the RF field at the interaction gap of each cavity. When the electron velocity is synchronized with the gap fields, its energy can be extracted at a multiple number of gaps. For a given total gap voltage, distributed interaction in an N-gap structure proportionally reduces the field level in the cavities as compared to that of a single gap structure, thus increasing the power handling capability by a factor up to N.sup.2. Tubes based on the extended interaction structure, such as the extended interaction oscillator (EIO) and extended interaction klystron (EIK), are often the choice for high average power operation, especially in the millimeter wave band. For example, EIOs are capable of kilowatt continuous wave (CW) power output in the Ku-band and hundreds of watts CW in the Ka-band. They are available, with reduced power, at much higher frequencies up to 260 GHz. Additional advantages of the extended interaction structure include a high gain-bandwidth product due to its larger resistance to quality (R/Q) value.
The extended interaction structure, on the other hand, does not by itself yield high efficiency. In linear electron tubes such as the klystron or EIK, ballistic bunching of electrons provides one of the most effective means for efficient beam-wave interaction. Conversion efficiencies in excess of 70% have been achieved in klystrons.
New applications, such as ceramic sintering and materials processing, are emerging which require moderately high power millimeter wave sources. These needs (for example, several kilowatts CW) are beyond the capability of state-of-the-art EIOs, which has motivated the development of gyrotrons for such applications. However, the gyrotron technology involves a complicated electron gun configuration and a bulky magnetic/power supply system. To fulfill the requirements of such moderate power applications, it is therefore desirable to provide a device having a greater interaction efficiency and increased power handling capability than does the EIO, but without the high cost and bulkiness of gyrotrons.